1. Field of the Invention
The present invention relates generally to a wireless communication system and, in particular, to a device and method for transmitting downlink control information in a wireless communication system in which the control information including downlink data transmission information, uplink resource allocation information, and transmission power control information is carried by a Physical Downlink Control Channel (PDCCH).
2. Description of the Related Art
In Long Term Evolution (LTE), multiple Downlink Control Information (DCI) formats are specified to support various transmission modes, and a User Equipment (UE) decodes the DCI using a blind decoding, wherein the DCI format type is determined by the size of the DCI format payload, rather than from any format identifier. Accordingly, the DCI formats have different sizes of payloads. The DCI formats include format 0, format 1, format 1A, etc. Therefore, when the DCI formats have the same size payload, the UE may fail to identify the DCI format correctly, and subsequently cannot decode the DCI.
In a conventional method for determining the payload size, the payload sizes of the DCI format 0 (N0) and DCI format 1A (N1A) are first determined and then the payload size of the DCI format 1 (N1) is determined. The payload size of the DCI format 1 is determined depending on the downlink bandwidth (i.e., a number of Resource Blocks (RBs)) and duplex mode, and by excluding padding bits and 16-bit UE identifier.
The DCI format 1 must have a payload size different from that of the DCI format 0/1A. If the number of information bits in DCI format 1 is equal to that in the DCI format 0/1A, a padding bit is appended to the DCI format 1 so as to update the N1=N1+1.
However, if the updated payload size of the DCI format 1 is an ambiguous size, another padding bit is appended to the DCI format 1 and N1A is incremented by 1. If the payload size of the DCI format 1 is not an ambiguous size, no further padding bits are necessary, and the payload size N1 of the DCI format 1 is determined.
However, the conventional DCI format 1 payload size determination method has a drawback in that the payload size of the DCI format 1 can be equal to that of another DCI format. As described above, a padding bit is appended to the DCI format 1 to differentiate the payload size of the DCI format 1 from that of the DCI format 0/1A, and another padding bit is further appended to the DCI format 1 such that the payload size of the DCI format 1 is not an ambiguous size. Therefore, although the payload size N1 of the DCI format is differentiated from payload size N0 of the DCI format 0, they may end up being equal to each other as a result of appending the padding bit to avoid the payload size N1 from belonging an ambiguous size.
For example, assuming that N0 and N1 have a relationship of N1=N0−1, the padding bit addition step is skipped because N0 and N1 have different values and, consequently, the relationship of N1=N0−1 is maintained. However, if N1 is an ambiguous size, a padding bit subsequently is appended to the DCI format 1 such that the N1 increments by 1 (N1+1), resulting in N0=N1.
Such a problem occurs when NRBDL=30 and NRBUL=6 in a Frequency Division Duplexing (FDD) mode. In this situation, the payload size of both the DCI format 0 and DCI format 1A is 25 bits and the payload size of the DCI format 1 is also 25 bits. Because the DCI format 1 cannot be differentiated from the DCI format 0/1A, based on the payload size, the UE fails to differentiate the two DCI formats in such a problematic situation, and Physical Downlink Control Channel (PDCCH) decoding fails.